Galaxy star formation rates (SFR), Hi masses, and H2 masses all show strong correlations with stellar mass for star-forming systems. Moreover, galaxies with high/low SFR also have high/low gas contents. Here we compare observations of correlated deviations off the main sequence from Janowiecki et al. to those in three galaxy formation simulations: Simba, EAGLE, and IllustrisTNG. Simulations predict that H2 deviations are linearly correlated with SFR deviations, as expected since star formation directly arises in dense gas. However, Hi deviations predictions vary, with Simba producing a near-linear slope but EAGLE and IllustrisTNG predicting sub-linear slopes. At earlier cosmic epochs, a sub-linear slope is always present in EAGLE and IllustrisTNG, but only at z = 2 in Simba, becoming linear at z ≤ 1. This suggests that the linear slope owes to the onset of AGN feedback, and hence that such feedback must remove Hi from the galaxy to match the observed linear slope. This provides unique insights into baryon cycling processes that connect gas within galaxies and their surroundings.

The study presented in this paper focuses on verifying cosmological simulations by searching for extended neutral hydrogen (HI) emission in the Fornax galaxy cluster. Using data from the MeerKAT Fornax Survey (MFS), which maps HI across the cluster, we compare observational results with predictions from the TNG50 cosmological simulations. The goal is to investigate the HI content of galaxies in dense environments, analyze gas stripping processes, and test the accuracy of simulations like TNG50 in modeling cold gas interactions. By comparing HI covering fractions and column densities, the study highlights both the successes and limitations of current models in representing the physical processes within galaxy clusters.

The advent of innovative new radio telescopes and instruments around the world has enabled HI observations of galaxies that are sharper, deeper, fainter and farther than ever before. This paper provides an overview of the extragalactic HI landscape today, with a focus on large surveys with SKA pathfinder facilities. Ongoing work to measure structural properties of HI disks from the WALLABY survey on the ASKAP telescope is also summarised, with an emphasis on the challenges of building statistical samples of the marginally detected and resolved systems that are characteristic of widefield untargetted surveys. This work paves the way to map HI disks across cosmic time with the SKA.

Atomic hydrogen (HI) is a vital player in the star-formation process in galaxies. It is the raw fuel for making molecules, an important shielding agent against interstellar radiation, and a buffer that soaks up the energy and momentum of stellar feedback. While for many years detailed studies of the HI thermal structure have been possible only in the Milky Way, the SKA pathfinders are expanding our view beyond the Solar neighborhood allowing for crucial tests of the HI heating and cooling processes under a wide range of physical conditions. This overview article highlights a few recent results and emphasizes areas where future observations can make large contributions. We show that the cold HI disk of the Milky Way is extended and flared, yet appears spatially coupled to the molecular gas. The cold neutral medium (CNM) in the Milky Way is colder and more abundant at higher optical extinctions due to more intense cooling and shielding. A comparison between the Milky Way, the Small Magellanic Cloud, the Large Magellanic Cloud and NGC 6822 shows a good agreement with predictions from recent numerical simulations on how the CNM fraction depends on metalliciity. The fraction and spatial distribution of the thermally unstable HI remain as open questions and observationally have not been studied beyond the Milky Way. The excitation temperature of the warm neutral medium (WNM) is not well understood. Observations suggest higher WNM temperatures than what is seen in numerical simulations. The SKA, once operational, will take the HI studies into a new era of dense HI absorption grids for the Milky Way and nearby galaxies, and the ability to study the HI thermal structure beyond the Local Group.

We present the results of our search for Hi emission from gas-rich galaxies in the Ophiuchus Supercluster region. The study covers a Galactic longitude range of 329° ≤ l ≤ 55° (excluding 358° ≤ l ≤ 2°) and a Galactic latitude range of |b| < 1.5°, within a redshift range of 7500 km s−1 < cz < 25000 km s-1. As part of the SARAO MeerKAT Galactic Plane Survey (SMKT-GPS), our primary objective is to map the large-scale structure of this region, investigate overdense areas, and explore possible connections between the Ophiuchus Supercluster and larger cosmic structures, such as the Great Attractor. Here we used 440 MeerKAT pointings, which were combined into 28 mosaics, each consisting of 22 pointings. These mosaics cover an area of approximately 252 deg2 with a median rms sensitivity of 0.46 mJy beam−1 per 44 km s−1 channel. Among the 589 Hi sources detected, 286 fall within the redshift range of 7500 km s−1 < cz < 12000 km s−1, which corresponds to the Ophiuchus Supercluster. Several overdense regions have been identified at redshifts of cz ∼ 9000, 11500, and 13500 km s−1.

Ultra diffuse galaxies (UDGs) are a low surface brightness population of galaxies exhibiting large sizes. Their recent unexpected abundance in clusters has brought into question our understanding of galaxy evolution and the effect of the harsh cluster environment. Using MeerKAT HI observations of the Hydra I cluster, a nearby Southern galaxy cluster with ongoing galaxy interactions and evolution, we explore the neutral hydrogen content in low stellar mass galaxies, specifically investigating UDGs. Detailed observations of these galaxies in HI provide insights into the processes they experience in these harsh and dense environments. We present preliminary results on our search for UDGs in the Hydra cluster with MeerKAT, including HI detections of a UDG near the center of the cluster, a possible LSB galaxy and a dark cloud of neutral hydrogen.

The MeerKAT Fornax Survey is performing a detailed study of the nearby Fornax galaxy cluster to understand how galaxies lose their cold gas and stop forming stars in low-mass clusters (Mvir ≤ 1014 M⊙). We are doing so through very deep (down to ∼1018 cm-2) and high resolution (up to ∼1 kpc and 1 km s-1) MeerKAT observations of neutral atomic hydrogen gas (HI) in a 1× 2 Mpc2 region centred on Fornax. At the time of writing, the survey is 88% complete. The data analyzed so far focus on the central region of the Fornax cluster and reveal, for the first time, the ubiquitous presence of tails and clouds of HI. Some of the HI is clearly being removed from Fornax galaxies as they interact with one another or with the intra-cluster medium. We present a sample of galaxies with long, one-sided, star-less HI tails (of which only one was previously known) radially oriented within the cluster. The properties of these tails represent the first unambiguous evidence of ram pressure shaping the distribution of HI in the Fornax cluster. Furthermore, interactions within the Fornax environment shape the HI mass function, the HI content of dwarf galaxies and determines how the HI sustains the nuclear activity of some cluster members and neighbouring galaxies.

The deep SARAO MeerKAT Galactic Plane Legacy Survey (SMGPS) covers the southern Milky Way along a narrow strip of ∆b ∼ 3°. While primarily aimed at exploring the Galaxy, the SMGPS provides a unique opportunity to trace the large-scale distribution of galaxies through the redshifted spectral 21cm line emission of their gas. The SMGPS H I survey is sensitive to galaxies with H I masses to 6·109 M⊙ throughout the probed volume (cz < 25 000 km s-1), and to 3·108 M⊙ at the Great Attractor distance. We will first summarize highlights of newly uncovered structures, then show preliminary results from deep MeerKAT64 follow-up observations of a hugely extended low H I mass galaxy disk identified in the SMGPS. The new observations, being sensitive to much lower H I column densities, trace the disk out even further (∼ 260 kpc).

The WALLABY HI survey on the Australian SKA Pathfinder is measuring Tully-Fisher distances and peculiar velocities for about 50,000 galaxies in the nearby universe. We have developed a new Bayesian methodology for simultaneously fitting the parameters of the Tully-Fisher relation and a model of the velocity field reconstructed from a redshift survey density field. This approach deals effectively with selection effects and biases and yields significantly more precise distances and peculiar velocities for individual galaxies. We demonstrate the methodology on the HI Tully-Fisher subsample of the CosmicFlows-4 dataset and obtain an improved estimate of the growth rate of structure in the present-day Universe from a comparison of the predicted and observed velocity fields, as well as an improved estimate of the Hubble constant. We simulate the HI Tully-Fisher observations from the WALLABY survey and forecast the constraints it will place on the growth rate of structure.

We present the first MeerKAT observations of the brightest neutral hydrogen (HI) cloud in the M31-M33 HI filament [Braun Thilker(2004)] at a resolution of 40″. Our analysis unveils previously unseen details in the internal kinematic structure of the HI cloud. We measure NHI ≈ 1019 cm-2 in the cloud, an order of magnitude greater than previously observed, and recover approximately two-thirds of the previously measured MHI. We see signs of ram pressure stripping implying a particle number density of nHI ≈ 10-4 cm-3 in the circumgalactic medium (CGM) of M31. The cloud may also have been tidally disrupted by M31. Both results suggest that the HI cloud is being destroyed as it falls through the CGM of M31.

Post-starburst galaxies (PSBs) are key to understanding how galaxies transition from active star formation to quiescence. We identify 36 PSBs by spectroscopic selection in the COSMOS field at z < 0.45. An analysis of their colours, star formation rates, and morphologies, suggests that they are in an early-stage post-starburst transitional phase. Additionally, we use neutral hydrogen and radio continuum data to investigate their gas content and their potential for star formation.

The observed star formation rates in nearby galaxies suggest they are replenishing their gas reservoir over cosmic time. Cosmological simulations predict that this gas accretion can happen in two modes: hot and cold. However, direct observation of the low-column density (∼ 1017 cm−2) neutral atomic hydrogen (Hi) tracing the cold mode remains unconfirmed. Using the the spectral stacking technique on ultra-deep (∼1018 cm−2) Hi data provided by the MHONGOOSE survey we still found no clear evidence for the presence of the low-column density gas around nearby galaxies as predicted by simulations. We suggests that the gaseous disk has a sharp cut-off at large distances, consequently, detecting H i in the circum-galactic and inter-galactic medium could remain challenging, even with future advanced radio telescopes.

Understanding what shapes the cold gas component of galaxies, which both provides the fuel for star formation and is strongly affected by the subsequent stellar feedback, is a crucial step towards a better understanding of galaxy evolution. This contribution will present detailed analysis of the atomic hydrogen (HI) in a sample of 46 Milky Way-mass galaxies from cosmological simulations. This set of simulations comprises galaxies evolved self-consistently across cosmic time with different baryonic sub-grid physics: three different star formation models and two different stellar feedback prescriptions. We discuss the complex interplay between star formation, feedback and HI properties of the galaxies and show that while the HI properties (mass, extent, density profile, scale height, morphology) of the simulated galaxies are in broad agreement with current observational results, they vary significantly between the different baryonic physics models. The morphology of HI discs is particularly sensitive to the different physics models.

In a poster presentation for IAU Symposium 392: “Neutral hydrogen in and around galaxies in the SKA era”, we gave an overview of the HI-MaNGA project which is working to obtain complementary information about the cold gas (neutral hydrogen traced by the radio 21cm line) content of Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) sample galaxies. MaNGA, part of the fourth incarnation of the Sloan Digital Sky Surveys (SDSS-IV), obtained spatially resolved spectral maps for 10,000 nearby galaxies selected to create a representative sample out of the SDSS Main Galaxy Sample. MaNGA data have provided a census of the stellar and ionized gas content of these galaxies, as well as kinematics of both stars and gas. Adding HI information via the HI-MaNGA program, which has observed or collected 21cm line data for 70% of the full MaNGA sample, has been crucial for a number of applications, but especially understanding the physical mechanisms that regulate gas accretion, and through that star formation and quenching of star formation.

The Small and Large Magellanic Clouds (SMC and LMC) provide perfect laboratories to study cold neutral medium (CNM) and warm neutral medium (WNM) phases of atomic hydrogen (HI) in low-metallicity environments. Recent GASKAP-HI survey using the ASKAP radio telescope shows detailed HI structure in these galaxies. This study focuses on the HI properties in the extreme outskirts of the SMC and LMC. We find that most CNM structures have spin temperatures below 40 K, colder than what is found for HI in the Milky Way. The CNM fraction in the SMC is below 20%, while some LMC regions exceed 70%. We identify a unique cold cloud in the LMC, entirely composed of the CNM, near a filamentary structure. Additionally, we find the presence of supergiant shells near most of the cold gas in the LMC outskirts, suggesting that large shell expansion likely promotes more cold cloud formation, compared to what is found in the SMC.

The distribution of neutral atomic hydrogen (H i) extends beyond galaxies into the circumgalactic (CGM) and intergalactic medium (IGM) due to its diffuse nature. Comprehensive mapping of H i gas on multiple scales requires combining the capabilities of both single-dish and interferometric radio telescopes. In this context, we introduce a FAST-MeerKAT synergy project aimed at mapping H i sources in the COSMOS field by leveraging the unparalleled sensitivity of FAST and the high resolution of MeerKAT, allowing for detailed studies of H i distribution across diverse galactic environments. Our observations demonstrate that the FAST-H i sources are marginally more H i massive than their MIGHTEE-H i counterparts by a median fraction of ∼ 7% for isolated galaxies and ∼ 18% for galaxy pairs, indicating that the majority of the H i gas in simple galactic environments is concentrated within galaxies, rather than dispersed throughout the surrounding CGM and IGM in our local Universe.

In this work, we peform a pilot study to investigate how the star formation rate history (SFRH) of galaxies relates to their neutral hydrogen gas (HI) mass. In particular we examine low redshift LADUMA galaxies and if this relation is dependent on redshift or galaxy morphologies. To do this we perform source finding over the LADUMA 0< z < 0.08 data cube, extract the multiwavelength photometry, then model the spectral energy distribution (SED) of each galaxy to obtain their SFRH.

We present the latest results from the First Large Absorption Survey in H i (FLASH), an untargeted survey searching for 21-cm line absorption within the redshift range of 0.4 < z < 1.0 using the ASKAP. 21-cm absorption provides a unique and unbiased probe of H i beyond the local Universe. The survey has successfully completed two phases of Pilot Surveys and commenced the Full Survey toward the wide-field southern sky with δ < +15 degrees and |b| > 8.5 degrees. Throughout the Pilot Surveys, we detected 30 genuine candidates of associated and intervening absorbers, revealing a diverse range of line properties across all redshifts. These absorbers are linked either to nearby host galaxies or distant background radio sources, respectively. We will highlight some of the newly discovered spectra and provide an update on the progress of the ongoing Full Survey, which will continue until 2028.

The aim of this research is to utilise data from the IMAGINE survey to probe deeper into the circumgalactic medium (CGM) of spiral galaxy NGC7424 and identify large scale HI structures using the high column densities achievable with ATCA. We intend to make systematic comparison of measurable HI properties such as HI flux and and linewidths of NGC7424 in both IMAGINE and MHONGOOSE (observed with MeerKAT) surveys as well as data obtained from single dish telescopes. The future goal is to combine both interferomentric and single dish data to obtain images with both high column density and resolution to search for evidence of cold mode accretion in the CGM.

In this analysis, we trace the turbulence properties in nearby spiral and dwarf galaxies and compare them with their star formation distribution. We use Gaussian decomposition of the velocity profiles of neutral atomic hydrogen (HI) and statistical tools from the TurbuStat package to trace the turbulence properties of our sample galaxies. We infer their local star formation rate using ultra-violet and infrared radiation as tracers. Our sample galaxies include spiral and dwarf galaxies from The HI Nearby Galaxy Surveys (THINGS). We aim to understand the possible effects of turbulence on the local star formation rate.